Say your going for 20 kills in a plane when you get to 10 you get your engine oil shot you have to go back and land and you have to end sortie because theres no repair pad.
i admit i like going on personal kill streaks sometimes for fun (yes some would call me a score ho, but seriously, i could care less about it or the name in lights unless its a squad thing... i like the challenge of killing 20 and landing) but this would take the challenge out of it for me...